1. Field of the Invention
The present invention relates to a particle therapy system for treating tumors such as cancers by irradiating the tumors with a proton beam or a heavy ion beam such as a carbon ion beam.
2. Description of the Related Art
A particle therapy, which irradiates a target volume (tumor volume) with an ion beam such as a proton beam or a carbon ion beam, is well known as one of the cancer treatment methods. When an ion beam (proton beam, carbon ion beam, etc.) with high energy enters a material, the ion beam loses much of its energy at the end of its propagation path (range). The particle therapy takes advantage of such a property of the ion beam and applies the ion beam to the patient so as to make the beam lose much of its energy in cancer cells. In the particle therapy, a dose distribution conformal to the shape of the target volume is formed by adjusting the spatial broadening (spatial distribution) and the energy of the ion beam.
A particle therapy system used for the particle therapy comprises an ion source for generating ions, an accelerator for accelerating the ions generated by the ion source and thereby forming an ion beam, a beam transport system for transporting the ion beam extracted from the accelerator, and an irradiation device for irradiating the target volume with the ion beam according to a desired dose distribution.
The accelerator used for the particle therapy system can be a synchrotron or a cyclotron, for example. The function of accelerating injected ions to a prescribed energy level and outputting the accelerated ions as an ion beam is common to the synchrotron and the cyclotron.
The ion beam extracted from the accelerator is transported by the beam transport system to the irradiation device. The beam transport system is equipped with bending electromagnets (hereinafter referred to as “magnet” or “magnets”) for changing the propagation direction of the ion beam, steering electromagnets (hereinafter referred to as “magnet” or “magnets”) for the fine adjustment of the beam propagation direction, and quadrupole electromagnets (hereinafter referred to as “magnet” or “magnets”) for giving convergence/divergence effects to the ion beam. By properly adjusting the levels of excitation of these magnets, a beam in an appropriate size and at an appropriate position can be transported to the irradiation device.
There are cases where the beam transport system and the irradiation device are mounted on a rotary gantry in order to irradiate the target volume with beams from multiple directions. The beam transport system of the particle therapy system having the rotary gantry can be roughly divided into a rotary beam transport system which is mounted on the rotary gantry and a fixed beam transport system which is mounted/fixed on the building.
The irradiation device forms an ion beam irradiation field that is conformal to the shape of the target volume. The irradiation field can be formed by two types of methods: a scatterer irradiation method and a scanning irradiation method. With the technological progress, the mainstream is shifting toward the scanning irradiation method capable of high-accuracy irradiation. An irradiation device employing the scanning irradiation method is equipped with two scanning electromagnets (hereinafter referred to as “magnet” or “magnets”) for scanning the beam. In order to irradiate exclusively the target volume, the beam in the irradiation device is scanned by these scanning magnets within a specified area orthogonal to the beam propagation direction. By successively changing the energy of the beam extracted from the accelerator, the reachable depth of the beam can be changed and the irradiation field conformal to the shape of the target volume can be formed.
Further, in the particle therapy, the reproducibility of the beam irradiation position is generally enhanced by performing initialization operation on the magnets to maintain the beam irradiation position accuracy at a high level.
A method for enhancing the accuracy and the reproducibility of the beam irradiation position has been described in JP-2005-296162-A. In this method, the energy level of the beam extracted from the accelerator is changed successively in order to form the irradiation field conformal to the shape of the target volume. To avoid ill effect of the hysteresis of the scanning magnets, a conversion table regarding conversion between beam position data detected by a beam position monitor and preset current values of the scanning magnets is stored in a storage device and the current values (electric current values) of the scanning magnets are set by using the stored conversion table according to beam position data determined based on treatment plan data.
JP-8-298200-A has described a method for uniformizing the remanent magnetization of the synchrotron magnets on each switching of the energy level in the operation successively changing the energy level of the ion beam. In this method, an initialization operation of temporarily increasing the excitation current value of each magnet (reexcitation) to a current value for the initialization (without shifting to the beam deceleration process immediately after the completion of the beam extraction at each energy level) and then demagnetizing each magnet is performed also on the magnets of the extraction beam transport system in the same way as the synchrotron magnets.